Severe Weather: How Ocean Storms Work, Part 1

BARBARA KLEIN: This is SCIENCE IN THE NEWS in VOA Special English. I'm Barbara Klein. BOB DOUGHTY:And I'm Bob Doughty. Today we remember Hurricane Katrina and tell about the science of severe ocean storms. BARBARA KLEIN: Many Americans are observing the fifth anniversary of one of the nation's worst natural disasters. Hurricane Katrina reached the state of Louisiana on the morning of August twenty-ninth, two thousand five. It was the costliest hurricane in American history, and one of the deadliest. Radio and television programs, concerts and films are recalling the storm and its effects on the nation. Literary readings and religious observances also are marking the event. Hurricane Katrina struck hardest in Louisiana, Mississippi and Alabama. Death and destruction from the hurricane and its effects extended along the Gulf Coast. More than one thousand eight hundred people were killed. BOB DOUGHTY:The storm formed over the Bahamas on August twenty-third, two thousand five. The next day, it grew strong enough for scientists to call it a tropical storm. Then it moved toward the United States. It first reached land in south Florida on August twenty-fifth. At that time, the National Hurricane Center said the winds were at a top continuing speed of more than one hundred thirty kilometers per hour. Experts identified the storm as a hurricane. They named it Katrina, and rated it as the least severe type of hurricane. Still, it caused flooding and killed people in Florida. BARBARA KLEIN: Hurricane Katrina weakened again after striking Florida. Later it moved to the Gulf of Mexico. The Gulf's warm waters helped it gain strength. At one point, the storm's winds were blowing at more than two hundred sixty-eight kilometers per hour. Experts increased its rating to the most severe hurricane. Time passed, and the winds again weakened. Then Hurricane Katrina reached land in Louisiana. Its speed had fallen to about two hundred kilometers per hour when it struck near New Orleans. But the wind was strong enough to pick up trees, vehicles and buildings. It threw them into the air like toys. Walls of water flooded over the land. Intense rain fell. Then Hurricane Katrina struck land again, this time at the border of Mississippi and Louisiana. Again, there was loss of life and terrible destruction. BOB DOUGHTY:Severe ocean storms in the northern part of the world usually develop in late summer or early autumn near the equator. Scientists call them cyclones when they develop over the Indian Ocean. When they happen over the northwestern Pacific Ocean, the storms are typhoons. And in the eastern Pacific and the Atlantic Ocean they are called hurricanes. Ocean storms develop when the air temperature in one area is different from the temperature nearby. Warmer air rises, while cooler air falls. These movements create a difference in the pressure of the atmosphere. BARBARA KLEIN: If the pressure changes over a large area, winds start to blow in a huge circle. High pressure air is pulled toward a low pressure center. Thick clouds form and heavy rains fall as the storm gains speed and moves over the ocean waters. Storms can get stronger as they move over warm ocean waters. The strongest, fastest winds of a hurricane blow in the area known as the eye wall. It surrounds the center, or eye, of the storm. The eye itself is calm by comparison. Wind speeds in severe ocean storms can reach more than two hundred fifty kilometers an hour. Up to fifty centimeters of rain can fall. Some storms have produced more than one hundred fifty centimeters of rain. These storms also cause high waves and ocean surges. A surge is a continuous movement of water that may reach as high as six meters or more. The water strikes low coastal areas. Surges are commonly responsible for about ninety percent of all deaths from ocean storms. Source: Voice of America